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Molecular emission observed during keV ion bombardment of solids
Nuclear Instruments and Methods 170 (1980) 585-589 © North-Holland Publishing Company
585
MOLECULAR EMISSION OBSERVED DURING keY ION BOMBARDMENT OF SOLIDS M. BRAUN and B. EMMOTH Research Institute of Physics, 104 05 Stockholm 50, Sweden
Visible and ultraviolet radiation observed during keY ion bombardment of solids has been interpreted as deexcitation radiation from molecules built up by atoms from the incoming beam (At), from the target material (Be, Nb, Mo), and from impurities (H, O). Molecular argon (At 2) is suggested to be formed from argon atoms implanted into a carbon target.
1. Introduction
2. Experimental
Continuum photon emission resulting from heavy ion bombardment o f solids has been observed for a number o f i o n - s o l i d interaction. The first kind arises when insulators are bombarded with ions. In this case the radiation continuum is caused by radiative recombination o f electrons with trapped holes in the bulk o f the solid [ 1 ]. Consequently this radiation can only be observed at the target surface but not in front o f the latter. The second type o f continuum, however, can be emitted in front o f the target surface, showing that the radiation in fact originates from sputtered material [ 1 - 7 ] . It is believed that this continuum is due to the radiative decay o f sputtered excited molecules or clusters ejected as a result o f collision processes occurring in the near-surface region. In the case o f molybdenum, niobium and tungsten it has been shown that the continua are associated with the presence of oxygen during thd b o m b a r d m e n t [7]. It should be mentioned that this optical emission is not known from conventional molecular spectroscopy. In this paper we present spectroscopic investigations o f this kind o f continuum radiation. The third type o f continuum radiation originates from beam particles which have been implanted and trapped in the target material. During the bombardment these particles can be released as excited molecules. In the present work, we report on the previously mentioned observations o f broad band spectral emission during argon b o m b a r d m e n t o f solids [3].
Energetic ions were produced in the 100 kV linear accelerator at this institute. After extraction, focusing and magnetic field mass separation, the ions were directed on to a solid sample. The target materials studied were Be, C, V, Cu, Zr, Nb, Mo, Ta and W. The ion induced collision phenomena were detected by optical means. In the wavelength region 2 0 0 - 7 0 0 nm, the light emitted from secondary particles passed through a quartz window and was then focused on to the entrance slit o f a scanning monochromator. The light in the wavelength region 1 0 5 - 2 0 0 nm, on the other hand, was detected with a LiF lens and a vacuum monochromator. In both cases single photon counting techniques were used. The initial pressure in the target chamber was less than 10 -7 Pa, although the gas pressure rose to about 5 X 10 -6 Pa during the iron irradiation. However, in the case o f vacuum ultraviolet photon detection, the pressure was 10 -4 Pa. Current densities in the range 1 0 - 5 0 p A/cm 2 were used and could be measured with a Faraday cup or directly on the target.
3. Results and discussion 3.1. The Ar2-continuum
During argon b o m b a r d m e n t o f a carbon target an intense continuum is observed in the vacuum ultraviolet with a maximum at X = 127.1 nm (see fig. 1). XIII. ELECTRON AND PHOTON EMISSION
586
M. Braun, B. Emmoth / Molecular emission
I
t.)
(3
o g ~g g_
120
130
140
150
~, (nrn)
Fig. 1. The second ultraviolet Ar2 continuum observed during 40 keV Ar÷ ion bombardment of a carbon target. The rest gas pressure is 10-4 Pa. This continuum is known as the second ultraviolet Ar2-continuum. In our experiment the first continuum is cut off by the LiF optics. These continua originate from deexcitation of either or both of the + lowest excited Ar2 states, 1 ~u(0u), 3~u(lu) and 3~(0u+), to the repulsive state, 1Eg(0~) [8,9]. Molecular argon has been observed in widely different experiments, e.g. in condensed discharges [10], during a-particle bombardment of liquid and solid Ar [11], and as stimulated emission in electron beam excited high-pressure argon [12]. More recently there have been a few reports in the literature for Ar2 molecule production during argon bombardment of a solid target [ 1 3 - 1 6 ] . By comparing the intensity of the CI lines with the Ar2 continuum it is obvious that the production rate o f excited Ar2 molecules is considerably higher than that of excited carbon atoms. No emission is found where the strong CO and C2 spectral band structures are expected, thus indicating the absence of the formation of these molecules in excited states. Furthermore we did not observe the characteristic Ar I or Ar II lines. The Ar2 continuum was also present when a copper target was bombarded with 40 keV Ar ÷ ions. The formation mechanism o f the excited Ar2 molecules is not completely understood at the present time. Experiments at low pressures (t7 < 1 atm) in discharges and beam excitation sources indicate that the molecules are formed in the radiating state in collisions involving one Ar atoms first excited to either of the 3pS4s levels (mainly the metastable 3pS4s (3/2)2 level) and two Ar atoms in the ground state [ 1 7 - 2 0 ] . According to Michaelson and Smith the
radiating states of the first and second continua are the vibrational levels v = 22 -+ 2 and v = 0, respectively, of the same l~u(0u~) state, which has a dissociation limit: 3pS4s(3/2)2 + 3p61S0 . Thonnard and Hurst [21], who used a 250 keV pulsed electron beam excitation, propose that a metastable Ar2 molecule is first formed in a three-body collision involving the 3pS4s'(1/2)l level, and this metastable molecule is then transferred to the radiating state in a new collision process. Another model of processes in a dense (~>1 atm) excited argon gas is also given by Lorentz [22]. The radiating molecules in our experiment may be created in a similar way as in discharges or beam excitations o f gaseous liquid, or solid argon. However, the continuum radiation, observed immediately after the Ar + beam is turned on, is very weak and the intensity increases slowly during the first minutes until the intensity becomes constant. This observation is made without any change in the bombarding ion beam intensity. When a previously Ar + implanted carbon target is bombarded with another noble gas, e.g. Kr ÷, the radiation continues, for a short time with rapidly decreasing intensity. These observations indicate that the continuous radiation originates from argon particles implanted into the carbon target. At the time when the intensity becomes constant, the maximum of implantation prof'de is reached, and the implanted Ar atom number density may then be as high as 1021 atoms/era a at the surface. It is therefore likely that the energy of an incoming ion beam will be transferred through collision cascades to previously implanted argon atoms and put them into motion. Some of these atoms will get a high enough speed component directed towards the surface in order to reach the surface. In an experiment, similar to the one described here, Gritsyna et. al. found molecular emission originating from He2 as a result of a formed carbon film on the target surface during 20 keV He + ion bombardment [23]. If two atoms, one in the ground state and the other excited to one o f the two metastable states, 3pS4s(3/2)2 and 3pS4s'(1/2)0, are close enough and with a total relative energy lower than the dissociation energy they may form a molecule. The natural lifetimes of these metastable states are about 1 second and the binding energy released at the molecular formation may be transferred to kinetic energy. How-
M. Braun, B. Emmoth / Molecular emission
587
I ta
2£0
260
280
300
320
3zO
X(nm)
Fig. 2. Broad band emission observed during 40 keV Ar ÷ ion b o m b a r d m e n t of Nb. The rest gas pressure is 10 -5 Pa.
ever, from the data presented here, it can not be concluded whether the molecular formation takes place at the surface or in front of it.
g.
290
300
310
I
I
I
320
330
3i.0
t
350 )-.(am)
Fig. 3. Spectrum observed during 40 keV Ar + ion bombardment of Mo. The rest gas pressure is 10 -5 Pa.
3.2. Other continua Some of the results presented in this section confirm previously reported data on broad-band emission in sputtering [ 1 - 7 ] . Particularly strong broad band emission is observed when Nb (max intensity at ?~= 252.7, 274.5, 304.5, 319.2 and 328.5 nm), Mo (max intensity at ?~= 295.8 and 346.5 nm), V, Zr, W and Ta are bombarded with keV inert-gas ions and the rest-gas pressure is ~10 -s Pa. An example of the Nb spectrum is shown in fig. 2. The observed continuous radiation is most likely due to molecular radiation and does not consist of closely spaced lines, although the resolution of our spectrometer is about 0.5 A. The intensity distribution of the broad band features in the wavelength region 2 0 0 - 7 0 0 nm is different for each metal and thus depending on the type of target, contrary to the observations reported by Kiyan et al. [4]. It is, however, independent of the type of beam ions. Our observations are similar to those shown in fig. 1 of ref. 2 of 8 keV Ar ÷ ion bombardment on Mo, W, Ta and the broad band radiation originates obviously from free particles with relatively long lifetimes since it is observed several millimeters in front of the targets. Atomic lines, mainly originating from transitions in the first spectrum of the sputtered metal atoms, are also present on the spectral scans of the radiation from the sputtered particles. Only atomic lines from the lowest ionization stages were observed in the wavelength region 2 2 0 - 6 0 0 nm. The spectrum obtained during 40 keV Ar ÷ ion bombardment of Mo is shown in fig. 3. The Mo I and Mo II lines are identi-
fled with the help of the atomic data given by Meggers et al. [24], and most of the observed lines can be identified as Mo I transitions. The Mo II lines are very weak if at all present and no Ar lines are seen. A similar spectrum as the one shown in fig. 3 was also obtained by the use of 100 keV Kr ÷ ion bombardment of Mo. The intensity of the broad band emission is rather independent of the beam energy in the region 1 0 200 keV. When the rest-gas pressure in the target chamber is below 10 -7 Pa, the continua disappear or become very weak for all the target materials which were studied in this work. This can be seen in fig. 4, which show part of a spectrum obtained when Nb is bombarded with 40 keV Ar÷ ions under UHV conditions. We have also found that the intensity of the broad band features relative to the atomic lines decreases considerably after a few minutes of ion bombardment of Mo and Nb. These observations are consistent with results found by Rausch et al. [7], and indicate that the production of the broad band emission decreases considerably when the oxide layer is sputtered away. On the basis of previous measurements [7] and our experimental results we suggest that the observed broad band features originate from deexcitation radiation of excited oxides or possibly homonuclear molecules, which are sputtered from the metal targets as neutrals. A definite identification is not possible as these molecular spectra are still not known from conventional spectroscopy. Considering the homonuclear molecules, they may be produced in their excited states mainly in the presence of the XIII. ELECTRON AND PHOTON EMISSION
588
M. Braun, B. Emmoth / Molecular emission
-.4r .~,r
I-~
14")
g,
O)
=== I
Fig. 4. Part of a line-rich Nb spectrum, obtained during 40 keV Ar÷ ion bombardment. The rest gas pressure was ~10 -7 Pa.
oxygen atoms. No lines originating from atomic or ionic oxygen have been observed. Furthermore, SIMS experiments show that the production of homonuclear particles is considerable in the sputtering process o f Nb [25], Mo [26], and W [27]. Although only charged particles are detected in these experiments, it is likely that also neutral dimers are created to a considerable extent during the sputtering process.
is)
~o
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-r
We also bombarded a Be target, detecting the optical emission from sputtered excited species. At UHV conditions strong line radiation from the first and second spectrum of Be is observed. When the total pressure in the target chamber is increased to 1 0 - 4 - 1 0 -s Pa, an additional strong Bell band spectrum at about 500 nm [28] is detected as well as the hydrogen Balmer series lines. This is shown in fig. 5. We explain this effect by the formation of Bell on the target surface which is sputtered in excited states, either as Bell or H. This is probably due to the high trapping efficiency of H20 at these pressures. The Bell band spectrum disappears at elevated target temperatures ( 4 0 0 - 5 0 0 K), indicating the correlation between the formation of Bell and the trapping of impurities. The intensity of the Balmer series lines behave in a similar way as the Bell band structure during annealing. 4. Conclusion
g
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I
500
i
520
~.(nm)
Fig. 5. The Bell band structure observed during ion bombardment of Be. The rest gas pressure is 10-s Pa.
We have found that a considerable production of molecules takes place when keV ions bombard solid surfaces. These molecules have their origin in the beam ions as well as in the target material and in impurities on the surface. The observed photon intensity close to the target surface during Ar ÷ ion bombardment, is dominated by radiation originating from Ar2 molecules. Our experiments indicate that the excited Ar2 molecules are formed during collisions with implanted argon atoms either at the surface or in front of it.
M. Braun, B. Emmoth / Molecular emission Strong band features observed during ion b o m b a r d m e n t o f Mo and Nb targets have been suggested to originate f r o m oxides or dimers o f these metals. Strong a t o m i c lines o f the target material are observed, the relative intensities o f which are different f r o m those reported f r o m observations in conventional l a b o r a t o r y light sources.
References [1] N.H. Tolk, D.L. Simms, E.B. Foley and C.W. White, Rad. Effects. 18 (1973) 221. [2] C.W. White, N.H. Tolk, J. Kraus and W.F. Van der Weg, Nucl. Instr. and Meth. 132 (1976) 419. [3] B. Emmoth, M. Braun and H.P. Palenius, J. Nucl. Mat. 63 (1976) 482. [4] T.S. Kiyan, V.V. Gritsyna and Ya.M. Foger, Nucl. Instr. and Meth. 132 (1976) 459. [5] C.B. Kerkdijk, K.H. Schartner, R. Kelly and F.W. Saris, Nucl. Instr. and Meth. 132 (1976) 427. [6] A.R. Bayly, P.J. Martin and R.J. McDonald, Nucl. Instr. and Meth. 132 (1976)459. [7] E.O. Rausch, A.I. Bazhin and E.W. Thomas, J. Chem. Phys. 65 (1976) 4447. [8] R.S. Mulliken, J. Chem. Phys. 52 (1970) 5170. [9] Y. Tanaka and K. Yoshino, J. Chem. Phys. 53 (1970) 2112. [10] R.E. Huffman, J.C. Larrabee and Y. Tanaka, Appl. Opt. 4 (1965) 1581. [11] J. Jorther, L. Meyer, S.A. Rice and E.G. Wilson, J. Chem. Phys. 42 (1965) 4250.
589
[12] W.M. Hughes, J. Shannon and R. Hunter, Appl. Phys. Lett. 24 (1974) 488. [13] H. Palenius, B. Emmoth, M. Braun and A. R6ste, Annual Report, Res. Inst. of Phys. (1976) p. 66. [14] C.W. White, E.W. Thomas, W.F. van der Weg and N.H. Tolk, in Inelastic ion-surface collisions, ed. N.H. Tolk, J.C. Tully, W. Heiland and C.W. White (Academic Press, New York, 1977) p. 201. [15] R.S. Bhattacharya, K.G. Lang, A. Scharman and K.H. Schartner, J. Phys. D: Appl. Phys. 11 (1978) 1935. [16] A.R. Knudson, D.J, Nagel, J. Comas and K.W. Hill, Nucl. Instr. and Meth. 149 (1978) 507. [17] E. Ellis and N.D. Tuiddy, J. Phys. B2 (1969) 1366. [18] W. Wieme and J. Wieme-Lenaerts, Phys. Lett. 47A (1974) 37. [19] P. Moerman, R. Bonciqu~ and P. Mortier, Phys. Lett. 49A (1974) 179. [20] R.C. Michaelson and A.L. Smith, J. Chem. Phys. 61 (1974) 2566. [21] N. Thonnard and G.S. Hurst, Phys. Rev. A5 (1972) 1110. [22] D.C. Lorentz, Physica 82C (1976) 19. [23] V.V. Gritsyna, T.S. Kiyan, A.G. Koval and Ya.M. Fogel, Zh. Eksp. Teor. Fiz. 58, No. 5 (1970) (Soy. Phys. JETP 58, No. 5 (1979)). [24] W.F. Meggers, C.H. Corliss and B.F. Scribner, Tables of spectral line intensities, Part 1, N.B.S. Monograph 145 (NBS Washington D.C., 1975). [25] J.W. Coburn, E.W. Eckstein and E. Kay, J. Vac. Sci. Technol. 12 (1975) 151. [26] G.A.v.d. Schootbrugge, A.G.J. de Wit and J.M. Fluit, Nucl. Instr. and Meth. 132 (1976) 321. [27] G. Staudemaier, Rad. Effects 18 (1973) 181. [28] E. Olsson, Z. Phys. 73 (1931) 732.
XIII. ELECTRON AND PHOTON EMISSION
585
MOLECULAR EMISSION OBSERVED DURING keY ION BOMBARDMENT OF SOLIDS M. BRAUN and B. EMMOTH Research Institute of Physics, 104 05 Stockholm 50, Sweden
Visible and ultraviolet radiation observed during keY ion bombardment of solids has been interpreted as deexcitation radiation from molecules built up by atoms from the incoming beam (At), from the target material (Be, Nb, Mo), and from impurities (H, O). Molecular argon (At 2) is suggested to be formed from argon atoms implanted into a carbon target.
1. Introduction
2. Experimental
Continuum photon emission resulting from heavy ion bombardment o f solids has been observed for a number o f i o n - s o l i d interaction. The first kind arises when insulators are bombarded with ions. In this case the radiation continuum is caused by radiative recombination o f electrons with trapped holes in the bulk o f the solid [ 1 ]. Consequently this radiation can only be observed at the target surface but not in front o f the latter. The second type o f continuum, however, can be emitted in front o f the target surface, showing that the radiation in fact originates from sputtered material [ 1 - 7 ] . It is believed that this continuum is due to the radiative decay o f sputtered excited molecules or clusters ejected as a result o f collision processes occurring in the near-surface region. In the case o f molybdenum, niobium and tungsten it has been shown that the continua are associated with the presence of oxygen during thd b o m b a r d m e n t [7]. It should be mentioned that this optical emission is not known from conventional molecular spectroscopy. In this paper we present spectroscopic investigations o f this kind o f continuum radiation. The third type o f continuum radiation originates from beam particles which have been implanted and trapped in the target material. During the bombardment these particles can be released as excited molecules. In the present work, we report on the previously mentioned observations o f broad band spectral emission during argon b o m b a r d m e n t o f solids [3].
Energetic ions were produced in the 100 kV linear accelerator at this institute. After extraction, focusing and magnetic field mass separation, the ions were directed on to a solid sample. The target materials studied were Be, C, V, Cu, Zr, Nb, Mo, Ta and W. The ion induced collision phenomena were detected by optical means. In the wavelength region 2 0 0 - 7 0 0 nm, the light emitted from secondary particles passed through a quartz window and was then focused on to the entrance slit o f a scanning monochromator. The light in the wavelength region 1 0 5 - 2 0 0 nm, on the other hand, was detected with a LiF lens and a vacuum monochromator. In both cases single photon counting techniques were used. The initial pressure in the target chamber was less than 10 -7 Pa, although the gas pressure rose to about 5 X 10 -6 Pa during the iron irradiation. However, in the case o f vacuum ultraviolet photon detection, the pressure was 10 -4 Pa. Current densities in the range 1 0 - 5 0 p A/cm 2 were used and could be measured with a Faraday cup or directly on the target.
3. Results and discussion 3.1. The Ar2-continuum
During argon b o m b a r d m e n t o f a carbon target an intense continuum is observed in the vacuum ultraviolet with a maximum at X = 127.1 nm (see fig. 1). XIII. ELECTRON AND PHOTON EMISSION
586
M. Braun, B. Emmoth / Molecular emission
I
t.)
(3
o g ~g g_
120
130
140
150
~, (nrn)
Fig. 1. The second ultraviolet Ar2 continuum observed during 40 keV Ar÷ ion bombardment of a carbon target. The rest gas pressure is 10-4 Pa. This continuum is known as the second ultraviolet Ar2-continuum. In our experiment the first continuum is cut off by the LiF optics. These continua originate from deexcitation of either or both of the + lowest excited Ar2 states, 1 ~u(0u), 3~u(lu) and 3~(0u+), to the repulsive state, 1Eg(0~) [8,9]. Molecular argon has been observed in widely different experiments, e.g. in condensed discharges [10], during a-particle bombardment of liquid and solid Ar [11], and as stimulated emission in electron beam excited high-pressure argon [12]. More recently there have been a few reports in the literature for Ar2 molecule production during argon bombardment of a solid target [ 1 3 - 1 6 ] . By comparing the intensity of the CI lines with the Ar2 continuum it is obvious that the production rate o f excited Ar2 molecules is considerably higher than that of excited carbon atoms. No emission is found where the strong CO and C2 spectral band structures are expected, thus indicating the absence of the formation of these molecules in excited states. Furthermore we did not observe the characteristic Ar I or Ar II lines. The Ar2 continuum was also present when a copper target was bombarded with 40 keV Ar ÷ ions. The formation mechanism o f the excited Ar2 molecules is not completely understood at the present time. Experiments at low pressures (t7 < 1 atm) in discharges and beam excitation sources indicate that the molecules are formed in the radiating state in collisions involving one Ar atoms first excited to either of the 3pS4s levels (mainly the metastable 3pS4s (3/2)2 level) and two Ar atoms in the ground state [ 1 7 - 2 0 ] . According to Michaelson and Smith the
radiating states of the first and second continua are the vibrational levels v = 22 -+ 2 and v = 0, respectively, of the same l~u(0u~) state, which has a dissociation limit: 3pS4s(3/2)2 + 3p61S0 . Thonnard and Hurst [21], who used a 250 keV pulsed electron beam excitation, propose that a metastable Ar2 molecule is first formed in a three-body collision involving the 3pS4s'(1/2)l level, and this metastable molecule is then transferred to the radiating state in a new collision process. Another model of processes in a dense (~>1 atm) excited argon gas is also given by Lorentz [22]. The radiating molecules in our experiment may be created in a similar way as in discharges or beam excitations o f gaseous liquid, or solid argon. However, the continuum radiation, observed immediately after the Ar + beam is turned on, is very weak and the intensity increases slowly during the first minutes until the intensity becomes constant. This observation is made without any change in the bombarding ion beam intensity. When a previously Ar + implanted carbon target is bombarded with another noble gas, e.g. Kr ÷, the radiation continues, for a short time with rapidly decreasing intensity. These observations indicate that the continuous radiation originates from argon particles implanted into the carbon target. At the time when the intensity becomes constant, the maximum of implantation prof'de is reached, and the implanted Ar atom number density may then be as high as 1021 atoms/era a at the surface. It is therefore likely that the energy of an incoming ion beam will be transferred through collision cascades to previously implanted argon atoms and put them into motion. Some of these atoms will get a high enough speed component directed towards the surface in order to reach the surface. In an experiment, similar to the one described here, Gritsyna et. al. found molecular emission originating from He2 as a result of a formed carbon film on the target surface during 20 keV He + ion bombardment [23]. If two atoms, one in the ground state and the other excited to one o f the two metastable states, 3pS4s(3/2)2 and 3pS4s'(1/2)0, are close enough and with a total relative energy lower than the dissociation energy they may form a molecule. The natural lifetimes of these metastable states are about 1 second and the binding energy released at the molecular formation may be transferred to kinetic energy. How-
M. Braun, B. Emmoth / Molecular emission
587
I ta
2£0
260
280
300
320
3zO
X(nm)
Fig. 2. Broad band emission observed during 40 keV Ar ÷ ion b o m b a r d m e n t of Nb. The rest gas pressure is 10 -5 Pa.
ever, from the data presented here, it can not be concluded whether the molecular formation takes place at the surface or in front of it.
g.
290
300
310
I
I
I
320
330
3i.0
t
350 )-.(am)
Fig. 3. Spectrum observed during 40 keV Ar + ion bombardment of Mo. The rest gas pressure is 10 -5 Pa.
3.2. Other continua Some of the results presented in this section confirm previously reported data on broad-band emission in sputtering [ 1 - 7 ] . Particularly strong broad band emission is observed when Nb (max intensity at ?~= 252.7, 274.5, 304.5, 319.2 and 328.5 nm), Mo (max intensity at ?~= 295.8 and 346.5 nm), V, Zr, W and Ta are bombarded with keV inert-gas ions and the rest-gas pressure is ~10 -s Pa. An example of the Nb spectrum is shown in fig. 2. The observed continuous radiation is most likely due to molecular radiation and does not consist of closely spaced lines, although the resolution of our spectrometer is about 0.5 A. The intensity distribution of the broad band features in the wavelength region 2 0 0 - 7 0 0 nm is different for each metal and thus depending on the type of target, contrary to the observations reported by Kiyan et al. [4]. It is, however, independent of the type of beam ions. Our observations are similar to those shown in fig. 1 of ref. 2 of 8 keV Ar ÷ ion bombardment on Mo, W, Ta and the broad band radiation originates obviously from free particles with relatively long lifetimes since it is observed several millimeters in front of the targets. Atomic lines, mainly originating from transitions in the first spectrum of the sputtered metal atoms, are also present on the spectral scans of the radiation from the sputtered particles. Only atomic lines from the lowest ionization stages were observed in the wavelength region 2 2 0 - 6 0 0 nm. The spectrum obtained during 40 keV Ar ÷ ion bombardment of Mo is shown in fig. 3. The Mo I and Mo II lines are identi-
fled with the help of the atomic data given by Meggers et al. [24], and most of the observed lines can be identified as Mo I transitions. The Mo II lines are very weak if at all present and no Ar lines are seen. A similar spectrum as the one shown in fig. 3 was also obtained by the use of 100 keV Kr ÷ ion bombardment of Mo. The intensity of the broad band emission is rather independent of the beam energy in the region 1 0 200 keV. When the rest-gas pressure in the target chamber is below 10 -7 Pa, the continua disappear or become very weak for all the target materials which were studied in this work. This can be seen in fig. 4, which show part of a spectrum obtained when Nb is bombarded with 40 keV Ar÷ ions under UHV conditions. We have also found that the intensity of the broad band features relative to the atomic lines decreases considerably after a few minutes of ion bombardment of Mo and Nb. These observations are consistent with results found by Rausch et al. [7], and indicate that the production of the broad band emission decreases considerably when the oxide layer is sputtered away. On the basis of previous measurements [7] and our experimental results we suggest that the observed broad band features originate from deexcitation radiation of excited oxides or possibly homonuclear molecules, which are sputtered from the metal targets as neutrals. A definite identification is not possible as these molecular spectra are still not known from conventional spectroscopy. Considering the homonuclear molecules, they may be produced in their excited states mainly in the presence of the XIII. ELECTRON AND PHOTON EMISSION
588
M. Braun, B. Emmoth / Molecular emission
-.4r .~,r
I-~
14")
g,
O)
=== I
Fig. 4. Part of a line-rich Nb spectrum, obtained during 40 keV Ar÷ ion bombardment. The rest gas pressure was ~10 -7 Pa.
oxygen atoms. No lines originating from atomic or ionic oxygen have been observed. Furthermore, SIMS experiments show that the production of homonuclear particles is considerable in the sputtering process o f Nb [25], Mo [26], and W [27]. Although only charged particles are detected in these experiments, it is likely that also neutral dimers are created to a considerable extent during the sputtering process.
is)
~o
oo
oo
-r
We also bombarded a Be target, detecting the optical emission from sputtered excited species. At UHV conditions strong line radiation from the first and second spectrum of Be is observed. When the total pressure in the target chamber is increased to 1 0 - 4 - 1 0 -s Pa, an additional strong Bell band spectrum at about 500 nm [28] is detected as well as the hydrogen Balmer series lines. This is shown in fig. 5. We explain this effect by the formation of Bell on the target surface which is sputtered in excited states, either as Bell or H. This is probably due to the high trapping efficiency of H20 at these pressures. The Bell band spectrum disappears at elevated target temperatures ( 4 0 0 - 5 0 0 K), indicating the correlation between the formation of Bell and the trapping of impurities. The intensity of the Balmer series lines behave in a similar way as the Bell band structure during annealing. 4. Conclusion
g
£
i 460
/,80
I
500
i
520
~.(nm)
Fig. 5. The Bell band structure observed during ion bombardment of Be. The rest gas pressure is 10-s Pa.
We have found that a considerable production of molecules takes place when keV ions bombard solid surfaces. These molecules have their origin in the beam ions as well as in the target material and in impurities on the surface. The observed photon intensity close to the target surface during Ar ÷ ion bombardment, is dominated by radiation originating from Ar2 molecules. Our experiments indicate that the excited Ar2 molecules are formed during collisions with implanted argon atoms either at the surface or in front of it.
M. Braun, B. Emmoth / Molecular emission Strong band features observed during ion b o m b a r d m e n t o f Mo and Nb targets have been suggested to originate f r o m oxides or dimers o f these metals. Strong a t o m i c lines o f the target material are observed, the relative intensities o f which are different f r o m those reported f r o m observations in conventional l a b o r a t o r y light sources.
References [1] N.H. Tolk, D.L. Simms, E.B. Foley and C.W. White, Rad. Effects. 18 (1973) 221. [2] C.W. White, N.H. Tolk, J. Kraus and W.F. Van der Weg, Nucl. Instr. and Meth. 132 (1976) 419. [3] B. Emmoth, M. Braun and H.P. Palenius, J. Nucl. Mat. 63 (1976) 482. [4] T.S. Kiyan, V.V. Gritsyna and Ya.M. Foger, Nucl. Instr. and Meth. 132 (1976) 459. [5] C.B. Kerkdijk, K.H. Schartner, R. Kelly and F.W. Saris, Nucl. Instr. and Meth. 132 (1976) 427. [6] A.R. Bayly, P.J. Martin and R.J. McDonald, Nucl. Instr. and Meth. 132 (1976)459. [7] E.O. Rausch, A.I. Bazhin and E.W. Thomas, J. Chem. Phys. 65 (1976) 4447. [8] R.S. Mulliken, J. Chem. Phys. 52 (1970) 5170. [9] Y. Tanaka and K. Yoshino, J. Chem. Phys. 53 (1970) 2112. [10] R.E. Huffman, J.C. Larrabee and Y. Tanaka, Appl. Opt. 4 (1965) 1581. [11] J. Jorther, L. Meyer, S.A. Rice and E.G. Wilson, J. Chem. Phys. 42 (1965) 4250.
589
[12] W.M. Hughes, J. Shannon and R. Hunter, Appl. Phys. Lett. 24 (1974) 488. [13] H. Palenius, B. Emmoth, M. Braun and A. R6ste, Annual Report, Res. Inst. of Phys. (1976) p. 66. [14] C.W. White, E.W. Thomas, W.F. van der Weg and N.H. Tolk, in Inelastic ion-surface collisions, ed. N.H. Tolk, J.C. Tully, W. Heiland and C.W. White (Academic Press, New York, 1977) p. 201. [15] R.S. Bhattacharya, K.G. Lang, A. Scharman and K.H. Schartner, J. Phys. D: Appl. Phys. 11 (1978) 1935. [16] A.R. Knudson, D.J, Nagel, J. Comas and K.W. Hill, Nucl. Instr. and Meth. 149 (1978) 507. [17] E. Ellis and N.D. Tuiddy, J. Phys. B2 (1969) 1366. [18] W. Wieme and J. Wieme-Lenaerts, Phys. Lett. 47A (1974) 37. [19] P. Moerman, R. Bonciqu~ and P. Mortier, Phys. Lett. 49A (1974) 179. [20] R.C. Michaelson and A.L. Smith, J. Chem. Phys. 61 (1974) 2566. [21] N. Thonnard and G.S. Hurst, Phys. Rev. A5 (1972) 1110. [22] D.C. Lorentz, Physica 82C (1976) 19. [23] V.V. Gritsyna, T.S. Kiyan, A.G. Koval and Ya.M. Fogel, Zh. Eksp. Teor. Fiz. 58, No. 5 (1970) (Soy. Phys. JETP 58, No. 5 (1979)). [24] W.F. Meggers, C.H. Corliss and B.F. Scribner, Tables of spectral line intensities, Part 1, N.B.S. Monograph 145 (NBS Washington D.C., 1975). [25] J.W. Coburn, E.W. Eckstein and E. Kay, J. Vac. Sci. Technol. 12 (1975) 151. [26] G.A.v.d. Schootbrugge, A.G.J. de Wit and J.M. Fluit, Nucl. Instr. and Meth. 132 (1976) 321. [27] G. Staudemaier, Rad. Effects 18 (1973) 181. [28] E. Olsson, Z. Phys. 73 (1931) 732.
XIII. ELECTRON AND PHOTON EMISSION